Documentation: gpio: capitalize GPIO in index title

Capitalise the title of the GPIO documentation page to match other
subsystems.
Signed-off-by: Kent Gibson <warthog618@gmail.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

Documentation/admin-guide/gpio/index.rst

 .. SPDX-License-Identifier: GPL-2.0
 
 ====
-gpio
+GPIO
 ====
 
 .. toctree::